Linux Tips • Page 71 of 115 • Getting you up to speed!

Disallow SSH Login For A Specific User

In today’s article, we’re going to learn how to disallow SSH login for a specific user. The reasons you might want to do this should be obvious, so that’ll save some time! Read on to learn how!

I’ve covered SSH in many articles. If you search for “SSH”, you’ll find a bunch of articles covering the subject. I’m not sure why there are so many, but there are. I seem to have a lot of notes on the subject.

Here, this link will help you search for SSH articles.

SSH is “Secure Shell”, a method to login to remote computers so that you can manage them without being their physically. It’s used by systems administrators regularly, without ever needing a GUI to manage their Linux systems.

It’s also used by people like me, too lazy to walk to the other side of the room. I’m literally using SSH to manage stuff on my laptop from here on this desktop as I write this. On top of that, while not logged in right now, I was using SSH to manage a VPS earlier today.

So, SSH isn’t just for professional system administrators and, if you use SSH at home, you might as well know how to secure it. This article will help you secure your system – by learning how to disallow SSH login for a specific user.

Disallow SSH Login For A Specific User:

This article requires an open terminal on (and connection to) the computer you wish to change. That may require you to login to that computer remotely. If you’re on a local device and you don’t know how to open the terminal, you can do so with your keyboard – just press CTRL + ALT + T and your default terminal should open. Otherwise, SSH into it like a boss.

With your terminal/connection now open, enter the following command:

sudo nano /etc/ssh/sshd_config

1	sudo nano /etc/ssh/sshd_config

Find a place to make a new line and enter the following with some care:

DenyUsers          <username>

1	DenyUsers <username>

Now, this one is a bit picky. Obviously, you substitute <username> with the real name – but in between “DenyUsers” and the username you absolutely MUST press the TAB key. If you try to just use spaces, it will not work! You MUST use the TAB key which will appear to insert spaces for you!

Assuming you’ve done everything correctly, you’ll need to restart SSH for the changes to take effect. You can do that with this command:

sudo systemctl restart ssh

1	sudo systemctl restart ssh

If you were logged into a remote system to make the changes on that system, the above command is gonna log you out and you’ll need to login again. You knew that, but I figure I’ll mention it.

Hmm… If you’re a barbarian that doesn’t use systemd, try this:

sudo service ssh restart

1	sudo service ssh restart

When SSH restarts, the prohibited user will get a “Permission Denied” message when they try to login. Ha! That’ll teach Jerry in accounting from thinking he’s a system admin!

Closure:

Whelp… You have another article. This one has shown you how to disable SSH login for a specific user (Jerry in accounting, who had no business accessing the server anyhow). You’re welcome!

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A Few Ways To Count CPU Processors Or Cores

So, in today’s article we’re going to learn a few ways to count CPU processors or cores. This is something you might already know about the system in front of you, but may want to verify with a remote system.

To the Linux system, there’s little difference between an individual processor or a core, or even a thread. Each core will appear as its own processor – and that’s exactly how it should be. Linux will treat each as its own processor and do its best to take advantage of multi-core/multi-CPU systems.

Now… Hmm… I shouldn’t need to explain this, but your computer has a processor called a CPU. It may have more than one, though that’s unlikely in a personal computer. Instead, your CPU may have multiple processors on it – each functioning more or less independently as ‘cores’. Additionally, your CPU cores may have multiple threads and each of those threads will appear as its own CPU.

That’s a very simplistic overview and I have to do things like limit how many characters are in the title of the article – and I don’t really want to try to explain everything.

For example, you could have a 4 core CPU that’s dual threaded. 4 x 2 = 8, so your system will see 8 CPUs. There’s only one physical processor, but you’re effectively working with 8 of them. (By the way, you can *sometimes* compile software optimized by the number of cores available.)

Count CPU Processors Or Cores:

(or threads, I suppose)

This article requires an open terminal, like many other articles on this site. If you don’t know how to open the terminal, you can do so with your keyboard – just press CTRL + ALT + T and your default terminal should open.

Now, there are so many ways that you can count the CPU processors or cores. We’re just going to cover a few of them. Feel free to leave other solutions as a comment. I’ll approve the comments as quickly as I can.

Solution #1

We just recently used the ‘cpuinfo‘ file to learn about your CPU. So, we’ll start with that one first. We’ll process it with ‘grep’ and then do some counting:

cat /proc/cpuinfo | grep processor | wc -l

1	cat /proc/cpuinfo \| grep processor \| wc -l

It might look something like this:

output showing there are 8 CPUs available — You’ll have to use your imagination for the rest. I’m only making one screenshot.

Now, those are individual threads that it’s counting – on the same physical CPU. But, to the system they appear to be individual CPUs. I can assure you, I do not have 8 physical CPUs in my laptop.

Solution #2:

Now, your computer does technically know if it’s a physical CPU, CPU core, or CPU core thread. It just normally doesn’t much care. If your CPU and OEM vendor did things properly, you can learn that information using ‘dmidecode‘.

They do not always implement this properly and you will sometimes find mistakes when using ‘dmidecode‘. Let’s hope for the best when we run this more convoluted command:

sudo dmidecode -t 4 | egrep -i 'core (count|enabled)|thread count|Version'

1	sudo dmidecode -t 4 \| egrep -i 'core (count\|enabled)\|thread count\|Version'

That one uses egrep and picks out multiple words to include. If you run that command, you should find out that your computer really can tell the difference between cores and threads – and physical CPU count (though not with this command).

Solution #3:

This final solution is nice and easy. If you’re gonna memorize one of these, this would be the one to memorize. In fact, if you’re going to need to count CPU processors or cores, you might just as well use this one. It’s easy and effective.

nproc --all

1	nproc --all

That one will output the total number of CPUs, cores, or threads. That’s all it will output, just a single number. That number is how many CPUs (effectively) your computer sees – and that’s it. It’s pretty much perfect.

Closure:

There you go. You have another article, this one explaining how to count CPU processors or cores (or threads, really). It’s handy information to know, though you probably already know it. Like I mentioned above, you can optimize C/C++ code to run on the specific number of cores – and there are other reasons you might want to know this.

One Way To See CPU Information

Today, we’re going to explore one way to see CPU information. On the original site, I had a couple of articles about this sort of stuff but we didn’t cover this method. It’s a short and sweet process, though I’ll show you a couple of ways to process the output.

The date that this will be published is July 4th. That’s a holiday in the United States. Not only do I live here, the vast majority of my readers live here (according to the stats I have). So, this will be a nice and easy article. For those of you who reside elsewhere, you’re welcome. You won’t have to work too hard today to understand what’s going on.

Anyhow, we’re not going to use any special tools. We’re not going to use any applications that you don’t already have installed. While we likely could, there really isn’t any need to. This being Linux, the information we’re after is already in a file. All we need to do is read that file.

So, let’s get to it…

How To See CPU Information:

With your terminal now open, let’s go ahead and read the file we need if we want to see the CPU information:

cat /proc/cpuinfo

1	cat /proc/cpuinfo

That’ll output a ton of text. You can read it as it is, or you can actually get it in a more readable form using the ‘less’ command. To do that, you just change out the ‘cat’ command, like so:

less /proc/cpuinfo

1	less /proc/cpuinfo

Now, just use the arrows on your keyboard to scroll down (or back up) through the text. When you want to close the output and return to the terminal, just press Q and it will close.

Closure:

That’s it. That’s all there is to this article that tells you how to see CPU information. It’s not a very tough article, and it’s quick. So, I met those goals! Yay me! Enjoy your holiday, even if the holiday is just a nice quick article that doesn’t require much thinking. Me? I’ll drink some beer and char some dead animal flesh.

Let’s Delete An Entry In Your Bash History

In today’s article, we’re going to learn how to delete an entry in your ‘bash’ history. It’s a useful skill to have, for any number of reasons. It’s not all that difficult and this shouldn’t be a very long article. Read on, my dear readers! Even a long-term Linux user might learn something – but I make no promises!

I’ve previously covered how to remove duplicates from your bash history. It may be worth checking that article out, as most of you are going to be using bash. Sure, there are other shells, but bash is the most common on desktops and servers. So, we might as well learn with bash.

If you don’t know, bash is both a language and an application. Unless otherwise specified, you’re almost certainly using bash when you open a terminal or TTY.

When you enter a command in the terminal or TTY, it’s saved to your bash history. In fact, it’s saved to the hidden file ~/.bash_history. If you so wanted, you could just open the .bash_history file with a text editor and remove lines as you wished. That works just fine.

However, in this article we’ll be using the ‘history’ command. It’s a handy command, useful for recalling previously entered commands and managing those stored commands. I guess this is really more an article about performing some very basic tasks with the history command. Like I said, it’s good for you to know this sort of stuff – especially as a Linux beginner.

Delete An Entry In Your Bash History:

So, with your handy-dandy freshly-cracked-open terminal, let’s just display your bash history. You’re going to do that with just the following command:

history

history

You’ll notice that every stored command from your bash history is assigned a number that’s shown on the left (or right, if you use a RTL language). Well, that’s how you delete it. You use that number in the following command:

history -d <command_number>

1	history -d <command_number>

You can use that command to delete an entry in your bash history, and you can do so as often as you’d like. While you’re there, you can also just plain clear your entire bash history with this command:

history -c

1	history -c

That will clear all of your bash history – more or less. Your current session may have not been written to the history at the time you issued that command. It might not be saved until you close the terminal instance. This makes it slightly more complicated to make sure the history is truly empty. However, it’s definitely close enough.

Anyhow, you might want to delete your bash history to remove commands that you’ve memorized. You might want to remove commands that didn’t work. If there are sensitive commands in your bash history, this is a way to remove them, surgically or en masse. There are all sorts of reasons why you might want to go through your bash history to delete past commands – and now you can!

Closure:

Yay! It’s another back-to-basics kind of article. In this one, we learn how to delete an entry in your bash history file. In some cases, it may be easier for you to just do so with a GUI and a GUI text editor. You don’t even need ‘sudo’ to make the changes and removing a lot of entries might be faster with a graphical application. Just remove the lines you don’t want showing up in the history any longer.

Also, I should mention somewhere that you can see the history by just pressing the up arrow, but I suspect folks will already know that. ‘Snot the best way to go about managing them, but you can see and select them, running them again as you wish.

Sort Files By Size (In The Terminal)

In today’s article, we’re going to cover something nice and simple; how to sort files by size in the terminal. This is something everyone should know, because sorting files is often a prerequisite to understanding and managing said files.

Besides, not all the articles have to be something complicated. The tagline for Linux-Tips is “Getting you up to speed!” It’s supposed to be aimed squarely at new Linux users. The problem is, many of those articles are boring to write and the 2nd largest group of readers aren’t really beginners.

So, yeah…

Today, we’ll have a nice, basic article that tells you how to sort files by size – in the terminal. In fact, some of the more regular users may not have these commands memorized. Now’s a good time to learn ’em.

We’ll be using the ‘ls’ command for this. It is said that you shouldn’t parse the output of ‘ls’ for anything important. It’s bad practice for reasons I think I’ve touched on before. However, you can safely use ‘ls’ for this process as it’s just sorting the files by size and doing so by itself.

For those that don’t know, ‘ls’ has a ton of options. It’s a tool used to show the contents of a directory. You can use man ls to get more information about the command. We’ve previously covered:

Let’s Use ‘ls’ To Sort Files By Time

Well, today we’ll use ‘ls’ to …

Sort Files By Size:

You might just as well stay right there in your default location – which should be your home directory. Feel free to switch to a new directory, but you really won’t need to. It’s an easy command.

First, we’ll show the output sorted to show the largest files first:

ls -lhS

ls -lhS

Of course, you can reverse that sort order and show the smallest files first. To do that, you just add -r (reverse) to your flags, like so:

ls -lhSr

ls -lhSr

That command should show you files listed with the smallest ones first and that’s really all there is to this article. Well, there’s the closure section – but nobody reads those.

Closure:

And, well, this particular closure section won’t have anything truly interesting or different in it. After all, this is just a simple article that shows you how to sort files by size. ‘Snot that much more to it. ‘Snot that much more that I can add.